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Ultraviolet and ionizing radiation for microorganism inactivation
ARTICLE IN PRESS
Water Research 38 (2004) 3940–3948 www.elsevier.com/locate/watres
Ultraviolet and ionizing radiation for microorganism inactivation Fariborz Taghipour,1 Chemical & Biological Engineering Department, University of British Columbia, 2357 Main Mall, Vancouver, British Columbia, Canada V6T 1Z4 Received 23 June 2003; received in revised form 28 May 2004; accepted 17 June 2004
Abstract The impacts of UV irradiation, gamma irradiation, and a combination of both on Escherichia coli inactivation in primary and secondary wastewater effluents were investigated. UV doses of 35 and 62 J/m2 were required for a 1-log inactivation of E. coli in the primary and secondary wastewater samples, respectively. A gamma dose of 170 Gy (J/kg) was required for a 1-log inactivation of E. coli in both wastewater samples. Variation in gamma radiation dose rates did not have a significant impact on the extent of inactivation at a given total dose. Gamma irradiation of previously UVirradiated samples indicated that particle-associated microorganisms, which are protected from UV, can be inactivated by ionizing radiation at a rate similar to that for free microorganism inactivation. An estimation of the energy required for disinfection indicated that, in general, the required energy and the energy cost for E. coli inactivation using ionizing radiation are considerably higher than those for UV radiation. r 2004 Elsevier Ltd. All rights reserved. Keywords: Microorganism inactivation; Disinfection; Ultraviolet; UV; Gamma; Ionizing radiation
1. Introduction Municipal wastewater generally requires disinfection to meet regulatory microbial limits. The main objective of disinfection is to reduce the concentration of waterborne pathogens to a level below the infective limit. To meet this objective, disinfection must inactivate a wide range of bacteria, viruses, and protozoa in a variety of wastewaters. Disinfection may be accomplished by chemical or physical means. An increasing awareness of the disadvantages of chemical disinfectants has resulted in the selection of ultraviolet (UV) radiation Tel.: (604) 822-1902; fax: (604) 8225407.
E-mail address: [email protected] (F. Taghipour). The research was performed at Trojan Technologies Inc., Canada. 1
as a promising alternative. In 1988, nearly 300 operating wastewater treatment plants were using UV disinfection (Bryan et al., 1992). The number of utilities using UV disinfection has increased considerably since then and is expected to increase significantly over the next decade (Schmelling, 2003). One of the factors affecting the performance of UV disinfection is the quality of the wastewater. The effectiveness of UV radiation for disinfecting high quality secondary or tertiary treated effluents has been demonstrated (e.g., USEPA, 1992; Blatchley et al., 1996; Braunstein et al., 1996; Oppenheimer et al., 1997). However, there has been uncertainty regarding the performance of UV radiation for the disinfection of marginal or poor quality effluents and primary treated wastewater (e.g., Scheible et al., 1986; Zukovs et al., 1986; Whitby and Palmateer, 1993; Sakamoto, 1997). A
0043-1354/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2004.06.016
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cause of this uncertainty is the presence of particleassociated microorganisms, which may have a negative impact on the disinfection process. This problem is not unique to UV disinfection. Destroying microorganisms within particles represents a problem for many other disinfection processes such as chlorine (Ormeci and Linden, 2002; Dietrich et al., 2003). High doses of disinfectant are usually required to expose microorganisms buried within the particles to lethal doses. Previous studies have shown that suspended particles in wastewater can increase microbial survival by shielding microorganisms from UV irradiation. Qualls et al. (1983) observed significant greater disinfection effect in filtered effluent than in unfiltered effluent. Liltved and Cripps (1999) reported improved overall bacteria removal from sea water using particle prefilters. Loge et al. (1999) concluded that UV can not penetrate particles by transmission through solid material. In another study (Loge et al., 2001), factors including the concentration of particles and the concentration of dispersed (non-particle associated) coliform bacteria were identified to influence the formation of particle-associated coliform. An study on the effect of particle size indicated that a minimum particle size governs the ability of a particle to shield coliform bacteria from UV light (Emerick et al., 2000). Ormeci and Linden (2002) found that naturally occurring particle-associated coliform survives at UV and chlorine disinfection doses typically applied in wastewater treatment plants. They reported that particle-associated coliform exhibits a slower inactivation rate and tailing, whereas non particle-associated coliform is more easily and rapidly inactivated. In their study, filtration was found to be effective in reducing particle-associated coliform and decreasing the total number of particles at all the particle sizes. Several investigations have reported relationship between suspended solid concentration and fecal coliform survival in UV irradiated wastewater samples (e.g. Whitby and Palmateer, 1993). The synergistic use of UV with other forms of particle-penetrating irradiation in an integrated disinfection process is a potential option for addressing this issue. Another type of energy, potentially useful for wastewater treatment, is ionizing radiation including high-energy electrons and gamma radiation. Studies on the use of electron beams in treating wastewater and municipal sewage sludge have been conducted at several locations around the world, including facilities in the United States, Canada, Japan, Austria, and France (International Atomic Energy Agency, 1990, 1997). These studies have demonstrated the effectiveness of electron beams for the destruction of organic contaminants and for the inactivation of pathogenic microorganisms. About 3-log inactivation of coliphage and total coliforms was reported at the dose of 5000 Gy (Farooq et al., 1993; Slifko et al., 1999). In another
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study (Pribil et al., 2000), electron beam doses of 2900, 520, 80, and 550 Gy were required to achieve 4-log inactivation of PHI 147, B40-8, MS-2, and E. coli, respectively. Gamma sources have also been used for municipal wastewater sludge treatment. Based on research funded in Germany in the 1970s, gamma irradiation was identified as an alternative method for disinfection (Lessel and Suess, 1984). In addition to the Geiselbullach liquid sludge irradiator in Germany, a number of gamma irradiation research facilities have been installed around the world to investigate wastewater and sludge treatment. Countries, including the United States, Canada, Japan, and India, have operated experimental facilities for waste irradiation investigation (IAEA, 1997). The effectiveness of gamma irradiation for organic chemical removal and pathogenic microorganism inactivation has been demonstrated. About 4-log inactivation of coliphage and total coliforms was reported at the dose of 5000 Gy (Farooq et al., 1993). In another study, 1-log inactivation of coliforms was achieved at the dose of 200 Gy, while for spore-forming bacteria, the dose required was in the order of 5000 Gy. Also, 3- and 4-log inactivation of coliform in raw sewage was achieved at 1000 and 2000 Gy radiation doses, respectively (Rawat et al., 1998). In a study by Thompson and Blatchley (2000), the inactivation rate constants for E. coli, MS-2, and Cryptosporidium parvum in air-equilibrium solutions were measured to be about 0.03, 0.0025, and 0.0006 Gy1, respectively. The inactivation rate constant of 0.03 corresponds to a dose of about 80 Gy for a 1-log E. coli inactivation. In another study (Sommer et al., 2001), gamma doses of 900, 610, 140, and 250 Gy were required to achieve 4-log inactivation of PHI 147, B40-8, MS-2, and E. coli, respectively. Table 1 summarizes some of the results of microorganism inactivation using ionizing irradiation. Many studies have demonstrated the effectiveness of each irradiation technology when used independently; however, no report was found in the open literature on the technical and economical feasibility of wastewater disinfection using UV radiation in conjunction with ionizing radiation, such as gamma rays or electron beams. UV light and ionizing radiation disinfect aqueous solutions in different ways. The effect of UV is due to a photochemical reaction initiated by the absorption of a photon by a molecular structure. Microorganisms are inactivated by UV light as a result of photochemical damage to nucleic acids. Ionizing radiation is believed to have primarily an indirect effect on microorganisms. Irradiation of aqueous material by gamma radiation or an electron beam produces highly reactive unstable intermediates such as hydroxyl radicals, hydrogen atoms, and hydrated electrons. These highly reactive intermediates can cause chemical changes in the aqueous system and within microorganisms,
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Table 1 Ionizing radiation of microorganism inactivation Treatment method
Microorganism
Dose (Gy)
Log inactivation
Reference
Electron Electron Electron Electron Electron Gamma Gamma Gamma Gamma Gamma Gamma
Total coliform PHI 147 B40-8 MS-2 E. coli Total coliform E. coli PHI 147 B40-8 MS-2 E. coli
5000 2900 520 80 550 5000 80 900 610 140 250
3 4 4 4 4 4 1 4 4 4 4
Farooq et al. (1993) Pribil et al. (2000) Pribil et al. (2000) Pribil et al. (2000) Pribil et al. (2000) Farooq et al. (1993) Thompson and Blatchley (2000) Sommer et al. (2001) Sommer et al. (2001) Sommer et al. (2001) Sommer et al. (2001)
beam beam beam beam beam
resulting in damage to the organisms in the system. The combination of UV and ionizing radiation for wastewater treatment may therefore provide complementary effects. Those microorganisms that are resistant to UV radiation by virtue of being buried within particles might be effectively eliminated by ionizing radiation. In this case, disinfection might be accomplished more efficiently if the UV is carried out at low doses to inactivate the free microorganisms, while ionizing radiation processes are used at an appropriate dose to destroy the particle-associated microorganisms, thereby avoiding the inefficient use of high UV doses. In this study, the impacts of UV irradiation, gamma irradiation, and a combination of both on the inactivation of E. coli, an indicator organism, were investigated in primary and secondary wastewater effluents. The inactivation sensitivity to UV and gamma irradiation varies among various types of parasites, bacteria, and viruses. The association of various microorganisms to particles is also different. Therefore, the results obtained from this study may not directly be applied to other microorganisms. High-energy electron beam irradiation yields similar water radiolysis products and results in similar water radiation chemistry as does gamma irradiation. The dose rate and distribution of the radiation within the irradiated volume, however, are different in gamma and electron beam irradiation. These may cause differences in the inactivation rates using these two irradiation technologies.
2. Experiments A UV collimated beam apparatus (Task Force on Wastewater Disinfection, Manual of Practice FD-10, 1996) containing a low-pressure mercury lamp (GPH460T5L/4, Trojan Technologies) was used for the UV irradiation of samples. The UV irradiance was measured with a calibrated radiometer (IL1700, International Light). Prior to each test, the UV transmittance
(the fraction of UV intensity transmitted through 1 cm path length of the sample) was measured using a UV spectrophotometer (P254C UV Photometer, Trojan Technologies). The concentration of total suspended solids (TSS) was measured according to the American Public Health Association (APHA) standard method 2540C (Clesceri et al., 1998). Samples, 50 mL in volume and 18 mm in depth, were irradiated in 60 36 mm sterile Petri dishes and were stirred continuously during the irradiation with a magnetic stir bar. The UV dose (J/m2) was calculated as the product of fluence rate (W/m2) and exposure time (s), and was corrected for UV transmittance of the samples. Gamma irradiation was performed by placing the samples inside the irradiation chamber of a Gammacell 220 (Nordion International). A Co-60 source with an average dose rate of 1.4 Gy/s was used as the radiation source. Samples, 100 mL in volume, were placed in 250-mL glass scintillation vials (air equilibrium) and were irradiated for the appropriate time to achieve the desired gamma irradiation dose. The E. coli concentration was measured using the APHA standard method 9222 (Clesceri et al., 1998) which involves membrane filtration. Serial dilutions of irradiated and control samples were performed. The samples were filtered, cultured on mFC-BCIG medium (Ciebin et al., 1995), and the number of colonies was counted after a 24-hour incubation period. The doseresponse curves were generated as a plot of the number of colony forming units (CFU) per 100 mL of the samples versus the applied dose of UV or gamma irradiation. Fecal coliforms were cultured on mFC medium and were analyzed using the same procedure (Clesceri et al., 1998) as the one used for E. coli. Effluent samples were obtained from the Greenway waste water treatment facility in London, Ontario, which uses primary sedimentation followed by secondary biological treatment. The effluent from the primary tanks flows to the aeration tanks where it is treated biologically to stabilize the dissolved and finely
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suspended impurities. The secondary treatment process employed is the activated sludge process and involves the use of bacteria and other minute organisms in the presence of air. The effluent from the aeration tanks passes to final tanks where the suspended solids settle out by gravity. The UV transmittances of the primary and secondary wastewater effluents were 47% and 72%, respectively. The total suspended solid in the primary and secondary wastewater effluents was 62 ppm and 8 ppm, respectively.
3. Results and discussion E. coli was inactivated when exposed to UV radiation, gamma radiation, and a combination of both. The inactivation rate of E. coli in all the cases was proportional to the radiation dose. When primary and secondary wastewater samples were exposed to UV irradiation, the E. coli count decreased progressively with an increase in UV dose (Fig. 1). Doses of 35 and 62 J/m2 were required for a 1-log inactivation of E. coli in the primary and secondary wastewaters, respectively. A UV dose of 30 J/m2 has been reported for 1-log E. coli inactivation (Wolfe, 1990). The rate of E. coli inactivation progressively decreased as irradiation proceeded, until it reached a ‘‘plateau’’ region (see Fig. 1). The E. coli count leveled off at a higher concentration in the primary effluent than in the secondary effluent. The difference observed in the required dose for 1-log E. coli inactivation in the primary and secondary wastewater samples might be due to the difference in the ratio of free to particle-associated E. coli in initial counts. The initial count is governed by the concentration of free microorganisms in the samples. As the initial concentration increases, the effect of particles on the inactivation rate diminishes. This is particularly valid for the wastewater samples where the ratio of free to particle-associated microorganisms is greater in primary
E-coli (CFU/100ml)
1000000 100000 10000 1000 100 10 1 0
100
200 300 Dose (mJ/cm2)
Secondary Effluent
400
500
Primary Effluent
Fig. 1. E. coli inactivation in primary and secondary wastewater effluent by UV irradiation (The error bars show 95% confidence intervals).
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compared to secondary effluent. Hence, the required dose for 1-log E. coli inactivation decreases with an increase in initial concentration. A quasi-empirical model, developed by Emerick et al. (2000) to describe the inactivation of coliform bacteria with UV in wastewater, can be used to describe the results. The UV disinfection model takes into account the effect of free and particle-associated microorganisms NðDÞ ¼ N f ekD þ
Np ð1 ekD Þ; KD
where NðDÞ is the number of surviving coliform bacteria after applied dose D; N f is the total number of free (non particle-associated) coliform bacteria, N p is the total number of particles that contain one or more coliform, D is the applied UV dose, and k is the inactivation rate constant. The first term in the equation represents the exponential degradation of free microorganisms. The second term represents the slow inactivation of particle-associated microorganisms. Therefore, the greater the ratio of free to particle-associated microorganism (primary effluent in this case), the higher the initial inactivation rate. Other possible explanation for the slower inactivation rate in secondary effluent could be the difference in the form of typical E. coli bacteria before and after going through the biological treatment. The plateau phenomenon is primarily due to the shielding of UV radiation from microorganisms within particles (Qualls et al., 1983). Free microorganisms and those at particle surfaces are readily disinfected, but interior microorganisms require longer exposure times (higher apparent doses) to be exposed to the same dose that would inactivate free microorganisms. The relatively slow inactivation of microorganisms associated with particulate material has been observed in other studies (Ormeci and Linden, 2002; Dietrich et al., 2003). Some of the microorganisms can be associated with particles to an extent that they are completely shielded from UV irradiation (Ormeci and Linden, 2002). The higher E. coli concentration in the plateau region of the primary effluent reflects the presence of a higher concentration of UV resistant particles in the primary effluent compared to the secondary effluent. This is likely the result of a higher concentration of suspended solids in the primary samples. The ratio of the E. coli count in the ‘‘plateau’’ regions of the primary and secondary samples (Fig. 1) is proportional to the ratio of the total suspended particles in the primary and secondary effluents. This is in agreement with the direct proportionality of fecal coliform survivor to suspended solids concentration in wastewater effluent, which is reported in other studies (Whitby and Palmateer, 1993; Qualls et al., 1983). In a separate study, the impact of particle size on the ease of disinfection was investigated. Primary wastewater samples were subjected to filtration through
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different pore sizes. The increased ease of disinfection with smaller particles sizes was reflected in the doseresponse curves of the filtrate samples (Fig. 2). Filtration of the wastewater samples at a specific probe size is expected to eliminate the particles larger than the probe size and reduce the possibility of particle-associated microorganisms presence in the sample. The improved overall bacterial removal efficiency by pre-filtration supports the hypothesis of bacteria associated with particles being protected from UV. Gamma irradiation of the samples showed a linear reduction in E. coli concentration by increasing the radiation dose. Unlike UV, gamma irradiation was effective at inactivating the particle-associated microorganisms, as complete inactivation of E. coli was achieved at higher doses (Fig. 3). A dose of 170 Gy was required for a 1-log inactivation of E. coli in both the primary and secondary effluents. Doses of 60–80 Gy for 1-log inactivation of E. coli samples in de-ionized water have been reported elsewhere (Sommer et al., 2001; Thompson and Blatchley, 2000). Therefore, the E. coli inactivation rate in wastewater effluent was lower
Fecal Coliforms (N/N0)
1.E+00 1.E-01 1.E-02 1.E-03 1.E-04 1.E-05 1.E-06 0
100
200
300
400
500
600
700
Dose (J/m2) Whole Effluent
53 Micron Filtrate
30 Micron Filtrate
20 Micron Filtrate
10 Micron Filtrate
Blended High Speed
Fig. 2. Fecal coliforms inactivation for filtered and unfiltered wastewater effluents.
than those reported for samples prepared with deionized water. The reason for the lower inactivation rate might be the existence of many other chemicals in the effluent samples. Those chemicals, in particular carbonates, bicarbonates, and other organic compounds can react with water radiolysis products such as hydroxyl radicals, decreasing their availability to reach microorganisms and hence lowering the apparent inactivation rates. The E. coli inactivation rates were similar in primary and secondary wastewater effluents. This might be explained by the nature of radical scavengers in wastewater before and after the biological treatment. The secondary biological treatment process employed is expected to oxidize some of the organic compounds in the wastewater. The concentration of many primary hydroxyl radical scavengers such as carbonates and bicarbonates, however, is not expected to change significantly as a result of biological treatment. The effect of gamma radiation dose rate on E. coli inactivation was investigated by the irradiation of the secondary effluent samples to a total dose of 125 Gy at different dose rates. The E. coli counts were reduced by 81.3% and 81.9% (from 5300 to 990 and 960 CFU/ 100 mL) after being exposed to the same total dose at radiation dose rates of 1.4 and 0.035 Gy/s, respectively. This indicated that the radiation dose rate did not have a significant impact on inactivation over the range that was examined. Previously UV-irradiated samples were irradiated subsequently by gamma radiation. The primary and secondary wastewater effluent samples were irradiated to various UV doses. The samples were then irradiated to a gamma dose of 250 Gy (Fig. 4 and Table 2). The E. coli inactivation rates of previously UV-irradiated samples were similar to those of non UV-irradiated samples. This indicated that particle-associated microorganisms, which are protected from UV, are being inactivated by ionizing radiation with a rate similar to the rate of free microorganism inactivation. This is not 1000000 E-coli (CFU/100ml)
E-coli (CFU/100ml)
1000000 100000 10000 1000 100 10
100000 10000 1000 100 10 1
1 0
200
400
600
800
1000
1200
Dose (kGy) Secondary Effluent
Primary Effluent
Fig. 3. E. coli inactivation in primary and secondary wastewater effluent by gamma irradiation. (The error bars show 95% confidence intervals).
0
50
100
150
200
250
300
Gamma Dose (Gy) UV (0 J/m2) + Gamma
UV (300 J/m2 ) + Gamma
Fig. 4. E. coli inactivation in primary wastewater effluent by UV irradiation (doses of 0 and 300 J/m2) followed by gamma irradiation (dose of 250 Gy).
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Table 2 E. coli inactivation in primary and secondary wastewater effluents by UV irradiation followed by gamma irradiation (dose of 250 Gy) Effluent
UV pre-Irradiation dose (J/m2)
Gamma Irradiation dose (Gy)
Initial–final concentrations (CFU/100 ml)
Required gamma dose for 1-log inactivation (Gy)
Primary Primary Secondary Secondary Secondary
0 300 0 50 150
250 250 250 250 250
790 000–22 000 78–2 5500–200 780–15 20–1
170 170 170 150 190
Table 3 E. coli inactivation in primary and secondary wastewater effluents by gamma irradiation (doses of 0, 125 and 250 Gy) followed by UV irradiation Effluent
Gamma pre-irradiation dose (Gy)
UV irradiation dose (J/m2)
Initial–final concentrations (CFU/100 ml)
Required UV dose for 1-log inactivation (J/m2)
Primary Primary Primary Secondary Secondary
0 125 250 0 250
100 100 100 150 150
650 000–1700 100000–400 2200–100 530–20 200–1
36 41 42 62 65
10000 E-coli (Geomean/100ml)
surprising, given that ionizing radiation has an indirect effect on microorganisms and suspended solid particles will not cause any shielding effect. Previously gamma-irradiated samples were irradiated subsequently by UV radiation. Primary and secondary wastewater effluent samples were irradiated to various gamma doses. The samples were then irradiated to UV doses of 100 or 150 J/m2 (Table 3). The E. coli inactivation rates were comparable to those of nongamma-irradiated samples. This showed that there was no longer a significant concentration of particle-associated microorganisms in the solution after gamma irradiation. It can be concluded that ionizing radiation inactivates particle-associated and free microorganisms simultaneously. A comparison of the energy required for E. coli inactivation in the secondary effluent by gamma and UV irradiation is provided in Fig. 5. It appears that E. coli can be inactivated at considerably lower energy per mass (or volume) of water using UV than can be accomplished using ionizing radiation. This could be explained by the direct effect of UV as opposed to the indirect effect of ionizing radiation on microorganisms. The following section will discuss the required energy for water disinfection using UV and ionizing radiation technologies.
1000 100 10 1 0
100
200
300
400
500
600
Energy (J/kg)
Gamma
UV
Fig. 5. Comparison of energy consumption per kg of wastewater for E. coli inactivation in secondary wastewater effluent using gamma and UV irradiation.
For an ideal reactor, the average UV dose Davg (J/m2) is related to average fluence rate E 0avg ðW=m2 Þ and average exposure time t (s) by Davg ¼ E 0avg t: The fluence rate as a function of the radial distance from the lamp, r (m), for an axisymmetric annular system, assuming the radiant power only emitted in radial direction, is given as I 0 T aðrRiÞ ; 2pr
4. Disinfection energy
E 0 ðrÞ ¼
The disinfection energy using UV is determined by the UV dose that is needed for a certain level of disinfection.
where I 0 is the UV lamp radiant power per unit length (W/m), T is the UV transmittance of the water, Ri is the
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radius of the UV lamp sleeve (m), and a (1/m) is the extinction coefficient multiplied by the concentration of the absorbing species. The average fluence rate for an annular reactor can be calculated as R 2p R Ro 0 E ðrÞr dr dy I 0 ðT aðRoRiÞ 1Þ E 0 avg ¼ 0 Ri 2 ¼ ; 2 ap lnðTÞðR2o R2i Þ pðRo Ri Þ where Ro is the radius of the reactor chamber. The average exposure time or average residence time a particle spends in the reactor is given as t¼
V ; Q
where V is the volume of the reactor and Q is the volumetric flow rate (m3/s). The average dose, Davg ; can be determined by multiplying average fluence rate by the average exposure time, i.e., Davg
I 0 ðT aðRoRiÞ 1ÞL ; ¼ aQ lnðTÞ
where L is the reactor length (m). In the limit as Ro ! 1; Davg2max ¼
I 0 L : aQ lnðTÞ
The power P (W) for disinfection would be P ¼ I 0 L ¼ Davg2max aQ lnðTÞ: Correction factors should be considered for hydrodynamically non-ideal reactors and when the UV lamp does not convert all input electrical energy to germicidal UV energy. A utilization factor ZUV,util can be defined, to accounts for the non-infinite reactor radius and nonuniform dose distribution, and a germicidal efficiency factor ZUV,eff to describe how much of the power transforms to germicidal UV light. Hence the true power required is P¼
Davg2max aQ lnðTÞ : ZUV;util ZUV;eff
ZUV,util depends on the UV reactor design and ZUV,eff on the UV lamp and ballast efficiency. For industrial UV reactors, a value of 0.7 for ZUV,util, and values of 0.3 and 0.1, respectively, for ZUV,eff for low and medium pressure mercury lamps, are reasonable. Assuming a dose of about 200 J/m2 (for 4-log inactivation of E. coli in primary wastewater in this experiment), the required energy for treating of water with 50% UV transmittance (for 1 cm) would be 8 102 kWh/m3. The disinfection energy using ionizing radiation is determined by the radiation dose D (Gy=J/kg) that is needed to decompose contaminants or inactivate microorganisms to a certain level, and on the utilization factor ZIR,util, which describes how much of the power, P (W), is effectively transferred into the water to be treated. These two parameters are related to each other and with
the water flow rate Q (m3/s) by the following equation P¼
DrQ ; ZIR;util
where r is the water density. The radiation dose D required for a certain water treatment process is a function of the chemical nature and concentration of the contaminants. The utilization factor ZIR,util depends on the design of the irradiation unit. In a commercial plant a value of 0.7 for ZIR,util can be considered (Bryan et al., 1992). Assuming a dose of about 650 Gy (for 4-log inactivation of E. coli in primary wastewater in this experiment), the required energy for treating the wastewater would therefore be 2.5 104 kWh/kg or 2.5 101 kWh/m3.
5. Disinfection energy cost The followings will provide an estimation of the disinfection energy cost (not a complete cost analysis) for UV and ionizing radiation. As stated in the previous section, the energies of 8 102 and 2.5 101 kWh/m3 are required, respectively, using UV and ionizing radiation for 4-log inactivation of E. coli in primary wastewater in this experiment. The energy cost for UV radiation and electron beam can be calculated by considering the electricity charge. Therefore, the aforementioned energy values correspond to the costs of about 0.4 and 1.25 b/m3, respectively, for UV radiation and electron beam, at an electricity price of 5 b/kWh. Additional costs should be considered for the electricity consumption of the necessary vacuum pumps for an electron beam. The energy cost for gamma radiation can be calculated by considering the price of radioactive material. If cobalt-60 is used as the radiation source, each disintegration will have energies of 2.5 MeV (cobalt-60 emits equal amounts of two gamma photons with energies of 1.17 and 1.33 MeV) or 4 1013 J. Therefore, the required disintegration per second or activity A (Bq) will be AðBqÞ ¼
P DrQ ¼ ; E g ð4 1013 ÞZIR;util
which can be rewritten to show the activity in curies (Ci) as AðCiÞ ¼
67:5DrQ : ZIR;util
With a value of 0.7 for ZIR,util, the cobalt-60 utilization efficiency, the activity will be AðCiÞ ¼ 96DrQ: About 13% of radioactive material will decay per year (cobalt-60 has a half-life of 5.27 years), therefore the
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Table 4 Disinfection energy and cost for 4 log of E. coli inactivation in primary wastewater using UV radiation, electron beam, and gamma radiation treatment (see the text for assumptions made) Treatment method
Dose
Energy (kWh/m3)
Energy cost
Treatment cost (b/m3)
UV Electron-beam Gamma
200 J/m2 650 Gy 650 Gy
8 102 2.5 101 2.5 101
5 b/kWh 5 b/kWh $1.0/Ci
0.4 1.25 25
activity replenishment required is Replenishment AðCi=yearÞ ¼ 12:5DrQ: A dose of about 650 kGy results in a cost of $6.5 107 for the cobalt-60 required to treat 1 m3/s of water, at a radioactive material cost of $1.0 per curie for cobalt-60. Cobalt-60 replenishment required for this flow rate would be $8.5 106 per year. This corresponds to a treatment price of about 25 b/m3. Table 4 summarizes the disinfection costs using different technologies. In all cases of UV radiation, electron beam and gamma radiation treatment, additional charges should be considered for the capital cost of the wastewater treatment plant itself as well as for the operational and maintenance (O&M) costs.
6. Conclusions Both UV and ionizing radiation are effective for E. coli inactivation in wastewater effluent. However, particle-associated microorganisms, which are protected from UV, can be inactivated by ionizing radiation. In general, the required energy and the energy cost for E. coli inactivation using ionizing radiation are considerably higher than those of UV radiation; although the required energy using ionizing radiation does not strongly depend on the water quality and its optical properties such as UV transmittance. If complete inactivation of microorganisms is desired, the use of UV, for inactivating the free microorganisms, followed by ionizing radiation, for inactivating particle-associated microorganisms, might be a promising option for minimizing the total required energy and cost.
Acknowledgement The author would like to acknowledge the support from Natural Science and Engineering Research Council of Canada (NSERC), Biology Laboratory at Trojan Technologies, and Ionizing Radiation Laboratory at the University of Toronto.
References Blatchley, E.R., Bastian, K.C., Duggirala, R.K., Alleman, J.E., Moore, M., Schuerch, P., 1996. Ultraviolet irradiation and chlorination/dechlorination for municipal wastewater disinfection. Water Environ. Res. 68 (2), 194–204. Braunstein, J.L., Loge, F.J., Tchobanoglous, G., Darby, J.L., 1996. Ultraviolet disinfection of filtrated activated sludge effluent for reuse applications. Water Environ. Res. 68 (2), 152–161. Bryan, E.H., Dick, R.I., Swinwood, J.F., Chairman, P.K., Carlos, D.A., Gerry, H., Waite, T., 1992. Radiation Energy Treatment of Water, Wastewater and Sludge. American Society of Civil Engineers, New York. Ciebin, B.W., Brodsky, M.H., Eddington, R., Horsnek, G., Choney, A., Palmateer, G., Ley, A., Joshi, R., Shears, G., 1995. Comparative evaluation of modified m-FC and m-TEC media for membrane filter enumeration of E. coli in water. Appl. Environ. Microbiol. 61 (11), 3940–3942. Clesceri, L.S., Greenberg, A.E., Eaton, A.D., (Eds.), 1998. Standard Methods for the Examination of Water and Wastewater, 20th ed., American Public Health Association, Washington, DC. Dietrich, J.P., Basagaoglu, H., Loge, F.J., Ginn, T.R., 2003. Preliminary assessment of transport processes influencing the penetration of chlorine into wastewater particles and the subsequent inactivation of particle-associated organisms. Water Res. 37 (1), 139–149. Emerick, R.W., Loge, F.J., Ginn, T.R., Darby, J.L., 2000. Modeling the inactivation of particle-associated coliform bacteria. Water Environ. Res. 72 (4), 432–438. Farooq, S., Kuruckz, C.N., Waite, D.W., Cooper, W., 1993. Disinfection of waste waters: high energy electron vs gamma irradiation. Water Res. 27 (7), 1177–1184. International Atomic Energy Agency, 1990. Technical Document, Proceedings of the Joint ASCE-IAEA Meeting on Radiation Treatment, Washington, DC, July 1990. International Atomic Energy Agency, 1997. Technical Document, GC(41)/INF/5. Lessel, T., Suess, A., 1984. Ten-year experience in operation of a sludge treatment plant using Gamma irradiation. Radn. Phys. Chem. J. 24, 3–16. Liltved, H., Cripps, S.J., 1999. Removal of particle-associated bacteria by prefiltration and ultraviolet irradiation. Aquacult. Res. 30, 445–450. Loge, F.J., Emerick, R.W., Thompson, D.E., Nelson, D.C., Darby, J.L., 1999. Factors influencing ultraviolet disinfection performance. Part I: light penetration to wastewater particles. Water Environ. Res. 71 (3), 377–381.
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F. Taghipour / Water Research 38 (2004) 3940–3948
Loge, F.J., Emerick, R.W., Ginn, T.R., Darby, J.L., 2001. Association of coliform bacteria with wastewater particles: impact of operational parameters of the activated sludge process. Water Res. 36 (1), 41–48. Oppenheimer, A.J., Jacangelo, J.G., Lane, J.M., Hoagland, J.E., 1997. Testing the Equivalency of Ultraviolet Light and Chlorine for Disinfection of Wastewater to Reclamation Standards. Water Environ. Res. 69, 14–24. Ormeci, B., Linden, K.G., 2002. Comparison of UV and chlorine inactivation of particle and non-particle associated coliform. Wat. Sci. Technol:Wat. Sup. 2 (5-6), 403–410. Qualls, R.G., Flynn, M.P., Johnson, J.D., 1983. The role of suspended particles in ultraviolet disinfection. J. Water Pollut. Control Fed. 55 (10), 1280–1285. Pribil, W., Sommer, R., Appelt, S., Gehringer, P., Eschweiler, H., Cabaj, H.L.A., Haider, T., 2000. Inactivation of Indicator Bacteria and Indicator Viruses by Ionizing Radiation, University of Vienna. Rawat, K.P., Sharma, A., Rao, S.M., 1998. Microbiological and Physical Analysis of Radiation Disinfected Municipal Sewage. Water Res. 32 (3), 737–740. Sakamoto, G., 1997. Clean Water for the 21st Century, Doing More for Less, UV Disinfection For Wastewater Reclamation, Proceedings of the 1997 PNPCA Annual Conference, Seattle, Washington, October, 1997. Scheible, O.K., Casey, M.C., Forndran, A., 1986. Ultraviolet Disinfection of Wastewater from Secondary Effluent and Combined Sewer Overflows. EPA/600/2-86/005, PB86-145182, 1986, US Environmental Protection Agency, Cincinnati, OH. Schmelling, D., 2003. Ultraviolet Disinfection Guidance Manual, EPA 815-D-03-007.
Slifko, T.R., Rose, J.B., Cooper, W.J., Nickelsen, M.J., 1999. Inactivation of Waterborne Microorganisms by a High-energy Electron Beam: a large scale study. International Symposium on Waterborne Pathogens, Proceedings, Milwaukee, WI, USA, August 29–September 1, pp. 112–127. Sommer, R., Pribil, W., Appelt, S., Gehringer, P., Eschweiler, H., Leth, H., Cabaj, A., Haider, T., 2001. Inactivation of bacteriophages in water by means of non-ionizing (UV-253.7 nm) and ionizing (Gamma) radiation: a comparative approach. Water Res. 35 (13), 3109–3116. Task Force on Wastewater Disinfection (Ed.), 1996. Ultraviolet Disinfection, in Wastewater Disinfection. Manual of Practice FD-10, 236-248. Water Environmental Federation, Alexandria, VA. Thompson, J.A., Blatchley, E.R., 2000. Gamma Irradiation for Inactivation of C. parvum, E. coli, and Coliphage MS-2. J. Environ. Eng. 126 (8), 761–768. USEPA, 1992. Ultraviolet Disinfection Technology Assessment. EPA 832-R-92-004, PB92-222868, US Environmental Protection Agency, Washington, DC. Whitby, G.E., Palmateer, G., 1993. The Effect of UV Transmission, Suspended Solids and Photoreactivation on Microorganisms in Wastewater Treated with UV Light. Water Sci. Technol. 27 (3–4), 379–386. Wolfe, R.L., 1990. Ultraviolet Disinfection of Potable Water: Current Technology and Research. Environ. Sci. Technol. 24 (6), 768–773. Zukovs, G., Kollar, J., Monteith, H.D., Ho, K.W.A., Ross, S.A., 1986. Disinfection of Low Quality Wastewaters by Ultraviolet Light Irradiation. J. Water Pollut. Control Fed. 58 (3), 199–206.
Water Research 38 (2004) 3940–3948 www.elsevier.com/locate/watres
Ultraviolet and ionizing radiation for microorganism inactivation Fariborz Taghipour,1 Chemical & Biological Engineering Department, University of British Columbia, 2357 Main Mall, Vancouver, British Columbia, Canada V6T 1Z4 Received 23 June 2003; received in revised form 28 May 2004; accepted 17 June 2004
Abstract The impacts of UV irradiation, gamma irradiation, and a combination of both on Escherichia coli inactivation in primary and secondary wastewater effluents were investigated. UV doses of 35 and 62 J/m2 were required for a 1-log inactivation of E. coli in the primary and secondary wastewater samples, respectively. A gamma dose of 170 Gy (J/kg) was required for a 1-log inactivation of E. coli in both wastewater samples. Variation in gamma radiation dose rates did not have a significant impact on the extent of inactivation at a given total dose. Gamma irradiation of previously UVirradiated samples indicated that particle-associated microorganisms, which are protected from UV, can be inactivated by ionizing radiation at a rate similar to that for free microorganism inactivation. An estimation of the energy required for disinfection indicated that, in general, the required energy and the energy cost for E. coli inactivation using ionizing radiation are considerably higher than those for UV radiation. r 2004 Elsevier Ltd. All rights reserved. Keywords: Microorganism inactivation; Disinfection; Ultraviolet; UV; Gamma; Ionizing radiation
1. Introduction Municipal wastewater generally requires disinfection to meet regulatory microbial limits. The main objective of disinfection is to reduce the concentration of waterborne pathogens to a level below the infective limit. To meet this objective, disinfection must inactivate a wide range of bacteria, viruses, and protozoa in a variety of wastewaters. Disinfection may be accomplished by chemical or physical means. An increasing awareness of the disadvantages of chemical disinfectants has resulted in the selection of ultraviolet (UV) radiation Tel.: (604) 822-1902; fax: (604) 8225407.
E-mail address: [email protected] (F. Taghipour). The research was performed at Trojan Technologies Inc., Canada. 1
as a promising alternative. In 1988, nearly 300 operating wastewater treatment plants were using UV disinfection (Bryan et al., 1992). The number of utilities using UV disinfection has increased considerably since then and is expected to increase significantly over the next decade (Schmelling, 2003). One of the factors affecting the performance of UV disinfection is the quality of the wastewater. The effectiveness of UV radiation for disinfecting high quality secondary or tertiary treated effluents has been demonstrated (e.g., USEPA, 1992; Blatchley et al., 1996; Braunstein et al., 1996; Oppenheimer et al., 1997). However, there has been uncertainty regarding the performance of UV radiation for the disinfection of marginal or poor quality effluents and primary treated wastewater (e.g., Scheible et al., 1986; Zukovs et al., 1986; Whitby and Palmateer, 1993; Sakamoto, 1997). A
0043-1354/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2004.06.016
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cause of this uncertainty is the presence of particleassociated microorganisms, which may have a negative impact on the disinfection process. This problem is not unique to UV disinfection. Destroying microorganisms within particles represents a problem for many other disinfection processes such as chlorine (Ormeci and Linden, 2002; Dietrich et al., 2003). High doses of disinfectant are usually required to expose microorganisms buried within the particles to lethal doses. Previous studies have shown that suspended particles in wastewater can increase microbial survival by shielding microorganisms from UV irradiation. Qualls et al. (1983) observed significant greater disinfection effect in filtered effluent than in unfiltered effluent. Liltved and Cripps (1999) reported improved overall bacteria removal from sea water using particle prefilters. Loge et al. (1999) concluded that UV can not penetrate particles by transmission through solid material. In another study (Loge et al., 2001), factors including the concentration of particles and the concentration of dispersed (non-particle associated) coliform bacteria were identified to influence the formation of particle-associated coliform. An study on the effect of particle size indicated that a minimum particle size governs the ability of a particle to shield coliform bacteria from UV light (Emerick et al., 2000). Ormeci and Linden (2002) found that naturally occurring particle-associated coliform survives at UV and chlorine disinfection doses typically applied in wastewater treatment plants. They reported that particle-associated coliform exhibits a slower inactivation rate and tailing, whereas non particle-associated coliform is more easily and rapidly inactivated. In their study, filtration was found to be effective in reducing particle-associated coliform and decreasing the total number of particles at all the particle sizes. Several investigations have reported relationship between suspended solid concentration and fecal coliform survival in UV irradiated wastewater samples (e.g. Whitby and Palmateer, 1993). The synergistic use of UV with other forms of particle-penetrating irradiation in an integrated disinfection process is a potential option for addressing this issue. Another type of energy, potentially useful for wastewater treatment, is ionizing radiation including high-energy electrons and gamma radiation. Studies on the use of electron beams in treating wastewater and municipal sewage sludge have been conducted at several locations around the world, including facilities in the United States, Canada, Japan, Austria, and France (International Atomic Energy Agency, 1990, 1997). These studies have demonstrated the effectiveness of electron beams for the destruction of organic contaminants and for the inactivation of pathogenic microorganisms. About 3-log inactivation of coliphage and total coliforms was reported at the dose of 5000 Gy (Farooq et al., 1993; Slifko et al., 1999). In another
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study (Pribil et al., 2000), electron beam doses of 2900, 520, 80, and 550 Gy were required to achieve 4-log inactivation of PHI 147, B40-8, MS-2, and E. coli, respectively. Gamma sources have also been used for municipal wastewater sludge treatment. Based on research funded in Germany in the 1970s, gamma irradiation was identified as an alternative method for disinfection (Lessel and Suess, 1984). In addition to the Geiselbullach liquid sludge irradiator in Germany, a number of gamma irradiation research facilities have been installed around the world to investigate wastewater and sludge treatment. Countries, including the United States, Canada, Japan, and India, have operated experimental facilities for waste irradiation investigation (IAEA, 1997). The effectiveness of gamma irradiation for organic chemical removal and pathogenic microorganism inactivation has been demonstrated. About 4-log inactivation of coliphage and total coliforms was reported at the dose of 5000 Gy (Farooq et al., 1993). In another study, 1-log inactivation of coliforms was achieved at the dose of 200 Gy, while for spore-forming bacteria, the dose required was in the order of 5000 Gy. Also, 3- and 4-log inactivation of coliform in raw sewage was achieved at 1000 and 2000 Gy radiation doses, respectively (Rawat et al., 1998). In a study by Thompson and Blatchley (2000), the inactivation rate constants for E. coli, MS-2, and Cryptosporidium parvum in air-equilibrium solutions were measured to be about 0.03, 0.0025, and 0.0006 Gy1, respectively. The inactivation rate constant of 0.03 corresponds to a dose of about 80 Gy for a 1-log E. coli inactivation. In another study (Sommer et al., 2001), gamma doses of 900, 610, 140, and 250 Gy were required to achieve 4-log inactivation of PHI 147, B40-8, MS-2, and E. coli, respectively. Table 1 summarizes some of the results of microorganism inactivation using ionizing irradiation. Many studies have demonstrated the effectiveness of each irradiation technology when used independently; however, no report was found in the open literature on the technical and economical feasibility of wastewater disinfection using UV radiation in conjunction with ionizing radiation, such as gamma rays or electron beams. UV light and ionizing radiation disinfect aqueous solutions in different ways. The effect of UV is due to a photochemical reaction initiated by the absorption of a photon by a molecular structure. Microorganisms are inactivated by UV light as a result of photochemical damage to nucleic acids. Ionizing radiation is believed to have primarily an indirect effect on microorganisms. Irradiation of aqueous material by gamma radiation or an electron beam produces highly reactive unstable intermediates such as hydroxyl radicals, hydrogen atoms, and hydrated electrons. These highly reactive intermediates can cause chemical changes in the aqueous system and within microorganisms,
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Table 1 Ionizing radiation of microorganism inactivation Treatment method
Microorganism
Dose (Gy)
Log inactivation
Reference
Electron Electron Electron Electron Electron Gamma Gamma Gamma Gamma Gamma Gamma
Total coliform PHI 147 B40-8 MS-2 E. coli Total coliform E. coli PHI 147 B40-8 MS-2 E. coli
5000 2900 520 80 550 5000 80 900 610 140 250
3 4 4 4 4 4 1 4 4 4 4
Farooq et al. (1993) Pribil et al. (2000) Pribil et al. (2000) Pribil et al. (2000) Pribil et al. (2000) Farooq et al. (1993) Thompson and Blatchley (2000) Sommer et al. (2001) Sommer et al. (2001) Sommer et al. (2001) Sommer et al. (2001)
beam beam beam beam beam
resulting in damage to the organisms in the system. The combination of UV and ionizing radiation for wastewater treatment may therefore provide complementary effects. Those microorganisms that are resistant to UV radiation by virtue of being buried within particles might be effectively eliminated by ionizing radiation. In this case, disinfection might be accomplished more efficiently if the UV is carried out at low doses to inactivate the free microorganisms, while ionizing radiation processes are used at an appropriate dose to destroy the particle-associated microorganisms, thereby avoiding the inefficient use of high UV doses. In this study, the impacts of UV irradiation, gamma irradiation, and a combination of both on the inactivation of E. coli, an indicator organism, were investigated in primary and secondary wastewater effluents. The inactivation sensitivity to UV and gamma irradiation varies among various types of parasites, bacteria, and viruses. The association of various microorganisms to particles is also different. Therefore, the results obtained from this study may not directly be applied to other microorganisms. High-energy electron beam irradiation yields similar water radiolysis products and results in similar water radiation chemistry as does gamma irradiation. The dose rate and distribution of the radiation within the irradiated volume, however, are different in gamma and electron beam irradiation. These may cause differences in the inactivation rates using these two irradiation technologies.
2. Experiments A UV collimated beam apparatus (Task Force on Wastewater Disinfection, Manual of Practice FD-10, 1996) containing a low-pressure mercury lamp (GPH460T5L/4, Trojan Technologies) was used for the UV irradiation of samples. The UV irradiance was measured with a calibrated radiometer (IL1700, International Light). Prior to each test, the UV transmittance
(the fraction of UV intensity transmitted through 1 cm path length of the sample) was measured using a UV spectrophotometer (P254C UV Photometer, Trojan Technologies). The concentration of total suspended solids (TSS) was measured according to the American Public Health Association (APHA) standard method 2540C (Clesceri et al., 1998). Samples, 50 mL in volume and 18 mm in depth, were irradiated in 60 36 mm sterile Petri dishes and were stirred continuously during the irradiation with a magnetic stir bar. The UV dose (J/m2) was calculated as the product of fluence rate (W/m2) and exposure time (s), and was corrected for UV transmittance of the samples. Gamma irradiation was performed by placing the samples inside the irradiation chamber of a Gammacell 220 (Nordion International). A Co-60 source with an average dose rate of 1.4 Gy/s was used as the radiation source. Samples, 100 mL in volume, were placed in 250-mL glass scintillation vials (air equilibrium) and were irradiated for the appropriate time to achieve the desired gamma irradiation dose. The E. coli concentration was measured using the APHA standard method 9222 (Clesceri et al., 1998) which involves membrane filtration. Serial dilutions of irradiated and control samples were performed. The samples were filtered, cultured on mFC-BCIG medium (Ciebin et al., 1995), and the number of colonies was counted after a 24-hour incubation period. The doseresponse curves were generated as a plot of the number of colony forming units (CFU) per 100 mL of the samples versus the applied dose of UV or gamma irradiation. Fecal coliforms were cultured on mFC medium and were analyzed using the same procedure (Clesceri et al., 1998) as the one used for E. coli. Effluent samples were obtained from the Greenway waste water treatment facility in London, Ontario, which uses primary sedimentation followed by secondary biological treatment. The effluent from the primary tanks flows to the aeration tanks where it is treated biologically to stabilize the dissolved and finely
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suspended impurities. The secondary treatment process employed is the activated sludge process and involves the use of bacteria and other minute organisms in the presence of air. The effluent from the aeration tanks passes to final tanks where the suspended solids settle out by gravity. The UV transmittances of the primary and secondary wastewater effluents were 47% and 72%, respectively. The total suspended solid in the primary and secondary wastewater effluents was 62 ppm and 8 ppm, respectively.
3. Results and discussion E. coli was inactivated when exposed to UV radiation, gamma radiation, and a combination of both. The inactivation rate of E. coli in all the cases was proportional to the radiation dose. When primary and secondary wastewater samples were exposed to UV irradiation, the E. coli count decreased progressively with an increase in UV dose (Fig. 1). Doses of 35 and 62 J/m2 were required for a 1-log inactivation of E. coli in the primary and secondary wastewaters, respectively. A UV dose of 30 J/m2 has been reported for 1-log E. coli inactivation (Wolfe, 1990). The rate of E. coli inactivation progressively decreased as irradiation proceeded, until it reached a ‘‘plateau’’ region (see Fig. 1). The E. coli count leveled off at a higher concentration in the primary effluent than in the secondary effluent. The difference observed in the required dose for 1-log E. coli inactivation in the primary and secondary wastewater samples might be due to the difference in the ratio of free to particle-associated E. coli in initial counts. The initial count is governed by the concentration of free microorganisms in the samples. As the initial concentration increases, the effect of particles on the inactivation rate diminishes. This is particularly valid for the wastewater samples where the ratio of free to particle-associated microorganisms is greater in primary
E-coli (CFU/100ml)
1000000 100000 10000 1000 100 10 1 0
100
200 300 Dose (mJ/cm2)
Secondary Effluent
400
500
Primary Effluent
Fig. 1. E. coli inactivation in primary and secondary wastewater effluent by UV irradiation (The error bars show 95% confidence intervals).
3943
compared to secondary effluent. Hence, the required dose for 1-log E. coli inactivation decreases with an increase in initial concentration. A quasi-empirical model, developed by Emerick et al. (2000) to describe the inactivation of coliform bacteria with UV in wastewater, can be used to describe the results. The UV disinfection model takes into account the effect of free and particle-associated microorganisms NðDÞ ¼ N f ekD þ
Np ð1 ekD Þ; KD
where NðDÞ is the number of surviving coliform bacteria after applied dose D; N f is the total number of free (non particle-associated) coliform bacteria, N p is the total number of particles that contain one or more coliform, D is the applied UV dose, and k is the inactivation rate constant. The first term in the equation represents the exponential degradation of free microorganisms. The second term represents the slow inactivation of particle-associated microorganisms. Therefore, the greater the ratio of free to particle-associated microorganism (primary effluent in this case), the higher the initial inactivation rate. Other possible explanation for the slower inactivation rate in secondary effluent could be the difference in the form of typical E. coli bacteria before and after going through the biological treatment. The plateau phenomenon is primarily due to the shielding of UV radiation from microorganisms within particles (Qualls et al., 1983). Free microorganisms and those at particle surfaces are readily disinfected, but interior microorganisms require longer exposure times (higher apparent doses) to be exposed to the same dose that would inactivate free microorganisms. The relatively slow inactivation of microorganisms associated with particulate material has been observed in other studies (Ormeci and Linden, 2002; Dietrich et al., 2003). Some of the microorganisms can be associated with particles to an extent that they are completely shielded from UV irradiation (Ormeci and Linden, 2002). The higher E. coli concentration in the plateau region of the primary effluent reflects the presence of a higher concentration of UV resistant particles in the primary effluent compared to the secondary effluent. This is likely the result of a higher concentration of suspended solids in the primary samples. The ratio of the E. coli count in the ‘‘plateau’’ regions of the primary and secondary samples (Fig. 1) is proportional to the ratio of the total suspended particles in the primary and secondary effluents. This is in agreement with the direct proportionality of fecal coliform survivor to suspended solids concentration in wastewater effluent, which is reported in other studies (Whitby and Palmateer, 1993; Qualls et al., 1983). In a separate study, the impact of particle size on the ease of disinfection was investigated. Primary wastewater samples were subjected to filtration through
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different pore sizes. The increased ease of disinfection with smaller particles sizes was reflected in the doseresponse curves of the filtrate samples (Fig. 2). Filtration of the wastewater samples at a specific probe size is expected to eliminate the particles larger than the probe size and reduce the possibility of particle-associated microorganisms presence in the sample. The improved overall bacterial removal efficiency by pre-filtration supports the hypothesis of bacteria associated with particles being protected from UV. Gamma irradiation of the samples showed a linear reduction in E. coli concentration by increasing the radiation dose. Unlike UV, gamma irradiation was effective at inactivating the particle-associated microorganisms, as complete inactivation of E. coli was achieved at higher doses (Fig. 3). A dose of 170 Gy was required for a 1-log inactivation of E. coli in both the primary and secondary effluents. Doses of 60–80 Gy for 1-log inactivation of E. coli samples in de-ionized water have been reported elsewhere (Sommer et al., 2001; Thompson and Blatchley, 2000). Therefore, the E. coli inactivation rate in wastewater effluent was lower
Fecal Coliforms (N/N0)
1.E+00 1.E-01 1.E-02 1.E-03 1.E-04 1.E-05 1.E-06 0
100
200
300
400
500
600
700
Dose (J/m2) Whole Effluent
53 Micron Filtrate
30 Micron Filtrate
20 Micron Filtrate
10 Micron Filtrate
Blended High Speed
Fig. 2. Fecal coliforms inactivation for filtered and unfiltered wastewater effluents.
than those reported for samples prepared with deionized water. The reason for the lower inactivation rate might be the existence of many other chemicals in the effluent samples. Those chemicals, in particular carbonates, bicarbonates, and other organic compounds can react with water radiolysis products such as hydroxyl radicals, decreasing their availability to reach microorganisms and hence lowering the apparent inactivation rates. The E. coli inactivation rates were similar in primary and secondary wastewater effluents. This might be explained by the nature of radical scavengers in wastewater before and after the biological treatment. The secondary biological treatment process employed is expected to oxidize some of the organic compounds in the wastewater. The concentration of many primary hydroxyl radical scavengers such as carbonates and bicarbonates, however, is not expected to change significantly as a result of biological treatment. The effect of gamma radiation dose rate on E. coli inactivation was investigated by the irradiation of the secondary effluent samples to a total dose of 125 Gy at different dose rates. The E. coli counts were reduced by 81.3% and 81.9% (from 5300 to 990 and 960 CFU/ 100 mL) after being exposed to the same total dose at radiation dose rates of 1.4 and 0.035 Gy/s, respectively. This indicated that the radiation dose rate did not have a significant impact on inactivation over the range that was examined. Previously UV-irradiated samples were irradiated subsequently by gamma radiation. The primary and secondary wastewater effluent samples were irradiated to various UV doses. The samples were then irradiated to a gamma dose of 250 Gy (Fig. 4 and Table 2). The E. coli inactivation rates of previously UV-irradiated samples were similar to those of non UV-irradiated samples. This indicated that particle-associated microorganisms, which are protected from UV, are being inactivated by ionizing radiation with a rate similar to the rate of free microorganism inactivation. This is not 1000000 E-coli (CFU/100ml)
E-coli (CFU/100ml)
1000000 100000 10000 1000 100 10
100000 10000 1000 100 10 1
1 0
200
400
600
800
1000
1200
Dose (kGy) Secondary Effluent
Primary Effluent
Fig. 3. E. coli inactivation in primary and secondary wastewater effluent by gamma irradiation. (The error bars show 95% confidence intervals).
0
50
100
150
200
250
300
Gamma Dose (Gy) UV (0 J/m2) + Gamma
UV (300 J/m2 ) + Gamma
Fig. 4. E. coli inactivation in primary wastewater effluent by UV irradiation (doses of 0 and 300 J/m2) followed by gamma irradiation (dose of 250 Gy).
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Table 2 E. coli inactivation in primary and secondary wastewater effluents by UV irradiation followed by gamma irradiation (dose of 250 Gy) Effluent
UV pre-Irradiation dose (J/m2)
Gamma Irradiation dose (Gy)
Initial–final concentrations (CFU/100 ml)
Required gamma dose for 1-log inactivation (Gy)
Primary Primary Secondary Secondary Secondary
0 300 0 50 150
250 250 250 250 250
790 000–22 000 78–2 5500–200 780–15 20–1
170 170 170 150 190
Table 3 E. coli inactivation in primary and secondary wastewater effluents by gamma irradiation (doses of 0, 125 and 250 Gy) followed by UV irradiation Effluent
Gamma pre-irradiation dose (Gy)
UV irradiation dose (J/m2)
Initial–final concentrations (CFU/100 ml)
Required UV dose for 1-log inactivation (J/m2)
Primary Primary Primary Secondary Secondary
0 125 250 0 250
100 100 100 150 150
650 000–1700 100000–400 2200–100 530–20 200–1
36 41 42 62 65
10000 E-coli (Geomean/100ml)
surprising, given that ionizing radiation has an indirect effect on microorganisms and suspended solid particles will not cause any shielding effect. Previously gamma-irradiated samples were irradiated subsequently by UV radiation. Primary and secondary wastewater effluent samples were irradiated to various gamma doses. The samples were then irradiated to UV doses of 100 or 150 J/m2 (Table 3). The E. coli inactivation rates were comparable to those of nongamma-irradiated samples. This showed that there was no longer a significant concentration of particle-associated microorganisms in the solution after gamma irradiation. It can be concluded that ionizing radiation inactivates particle-associated and free microorganisms simultaneously. A comparison of the energy required for E. coli inactivation in the secondary effluent by gamma and UV irradiation is provided in Fig. 5. It appears that E. coli can be inactivated at considerably lower energy per mass (or volume) of water using UV than can be accomplished using ionizing radiation. This could be explained by the direct effect of UV as opposed to the indirect effect of ionizing radiation on microorganisms. The following section will discuss the required energy for water disinfection using UV and ionizing radiation technologies.
1000 100 10 1 0
100
200
300
400
500
600
Energy (J/kg)
Gamma
UV
Fig. 5. Comparison of energy consumption per kg of wastewater for E. coli inactivation in secondary wastewater effluent using gamma and UV irradiation.
For an ideal reactor, the average UV dose Davg (J/m2) is related to average fluence rate E 0avg ðW=m2 Þ and average exposure time t (s) by Davg ¼ E 0avg t: The fluence rate as a function of the radial distance from the lamp, r (m), for an axisymmetric annular system, assuming the radiant power only emitted in radial direction, is given as I 0 T aðrRiÞ ; 2pr
4. Disinfection energy
E 0 ðrÞ ¼
The disinfection energy using UV is determined by the UV dose that is needed for a certain level of disinfection.
where I 0 is the UV lamp radiant power per unit length (W/m), T is the UV transmittance of the water, Ri is the
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radius of the UV lamp sleeve (m), and a (1/m) is the extinction coefficient multiplied by the concentration of the absorbing species. The average fluence rate for an annular reactor can be calculated as R 2p R Ro 0 E ðrÞr dr dy I 0 ðT aðRoRiÞ 1Þ E 0 avg ¼ 0 Ri 2 ¼ ; 2 ap lnðTÞðR2o R2i Þ pðRo Ri Þ where Ro is the radius of the reactor chamber. The average exposure time or average residence time a particle spends in the reactor is given as t¼
V ; Q
where V is the volume of the reactor and Q is the volumetric flow rate (m3/s). The average dose, Davg ; can be determined by multiplying average fluence rate by the average exposure time, i.e., Davg
I 0 ðT aðRoRiÞ 1ÞL ; ¼ aQ lnðTÞ
where L is the reactor length (m). In the limit as Ro ! 1; Davg2max ¼
I 0 L : aQ lnðTÞ
The power P (W) for disinfection would be P ¼ I 0 L ¼ Davg2max aQ lnðTÞ: Correction factors should be considered for hydrodynamically non-ideal reactors and when the UV lamp does not convert all input electrical energy to germicidal UV energy. A utilization factor ZUV,util can be defined, to accounts for the non-infinite reactor radius and nonuniform dose distribution, and a germicidal efficiency factor ZUV,eff to describe how much of the power transforms to germicidal UV light. Hence the true power required is P¼
Davg2max aQ lnðTÞ : ZUV;util ZUV;eff
ZUV,util depends on the UV reactor design and ZUV,eff on the UV lamp and ballast efficiency. For industrial UV reactors, a value of 0.7 for ZUV,util, and values of 0.3 and 0.1, respectively, for ZUV,eff for low and medium pressure mercury lamps, are reasonable. Assuming a dose of about 200 J/m2 (for 4-log inactivation of E. coli in primary wastewater in this experiment), the required energy for treating of water with 50% UV transmittance (for 1 cm) would be 8 102 kWh/m3. The disinfection energy using ionizing radiation is determined by the radiation dose D (Gy=J/kg) that is needed to decompose contaminants or inactivate microorganisms to a certain level, and on the utilization factor ZIR,util, which describes how much of the power, P (W), is effectively transferred into the water to be treated. These two parameters are related to each other and with
the water flow rate Q (m3/s) by the following equation P¼
DrQ ; ZIR;util
where r is the water density. The radiation dose D required for a certain water treatment process is a function of the chemical nature and concentration of the contaminants. The utilization factor ZIR,util depends on the design of the irradiation unit. In a commercial plant a value of 0.7 for ZIR,util can be considered (Bryan et al., 1992). Assuming a dose of about 650 Gy (for 4-log inactivation of E. coli in primary wastewater in this experiment), the required energy for treating the wastewater would therefore be 2.5 104 kWh/kg or 2.5 101 kWh/m3.
5. Disinfection energy cost The followings will provide an estimation of the disinfection energy cost (not a complete cost analysis) for UV and ionizing radiation. As stated in the previous section, the energies of 8 102 and 2.5 101 kWh/m3 are required, respectively, using UV and ionizing radiation for 4-log inactivation of E. coli in primary wastewater in this experiment. The energy cost for UV radiation and electron beam can be calculated by considering the electricity charge. Therefore, the aforementioned energy values correspond to the costs of about 0.4 and 1.25 b/m3, respectively, for UV radiation and electron beam, at an electricity price of 5 b/kWh. Additional costs should be considered for the electricity consumption of the necessary vacuum pumps for an electron beam. The energy cost for gamma radiation can be calculated by considering the price of radioactive material. If cobalt-60 is used as the radiation source, each disintegration will have energies of 2.5 MeV (cobalt-60 emits equal amounts of two gamma photons with energies of 1.17 and 1.33 MeV) or 4 1013 J. Therefore, the required disintegration per second or activity A (Bq) will be AðBqÞ ¼
P DrQ ¼ ; E g ð4 1013 ÞZIR;util
which can be rewritten to show the activity in curies (Ci) as AðCiÞ ¼
67:5DrQ : ZIR;util
With a value of 0.7 for ZIR,util, the cobalt-60 utilization efficiency, the activity will be AðCiÞ ¼ 96DrQ: About 13% of radioactive material will decay per year (cobalt-60 has a half-life of 5.27 years), therefore the
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Table 4 Disinfection energy and cost for 4 log of E. coli inactivation in primary wastewater using UV radiation, electron beam, and gamma radiation treatment (see the text for assumptions made) Treatment method
Dose
Energy (kWh/m3)
Energy cost
Treatment cost (b/m3)
UV Electron-beam Gamma
200 J/m2 650 Gy 650 Gy
8 102 2.5 101 2.5 101
5 b/kWh 5 b/kWh $1.0/Ci
0.4 1.25 25
activity replenishment required is Replenishment AðCi=yearÞ ¼ 12:5DrQ: A dose of about 650 kGy results in a cost of $6.5 107 for the cobalt-60 required to treat 1 m3/s of water, at a radioactive material cost of $1.0 per curie for cobalt-60. Cobalt-60 replenishment required for this flow rate would be $8.5 106 per year. This corresponds to a treatment price of about 25 b/m3. Table 4 summarizes the disinfection costs using different technologies. In all cases of UV radiation, electron beam and gamma radiation treatment, additional charges should be considered for the capital cost of the wastewater treatment plant itself as well as for the operational and maintenance (O&M) costs.
6. Conclusions Both UV and ionizing radiation are effective for E. coli inactivation in wastewater effluent. However, particle-associated microorganisms, which are protected from UV, can be inactivated by ionizing radiation. In general, the required energy and the energy cost for E. coli inactivation using ionizing radiation are considerably higher than those of UV radiation; although the required energy using ionizing radiation does not strongly depend on the water quality and its optical properties such as UV transmittance. If complete inactivation of microorganisms is desired, the use of UV, for inactivating the free microorganisms, followed by ionizing radiation, for inactivating particle-associated microorganisms, might be a promising option for minimizing the total required energy and cost.
Acknowledgement The author would like to acknowledge the support from Natural Science and Engineering Research Council of Canada (NSERC), Biology Laboratory at Trojan Technologies, and Ionizing Radiation Laboratory at the University of Toronto.
References Blatchley, E.R., Bastian, K.C., Duggirala, R.K., Alleman, J.E., Moore, M., Schuerch, P., 1996. Ultraviolet irradiation and chlorination/dechlorination for municipal wastewater disinfection. Water Environ. Res. 68 (2), 194–204. Braunstein, J.L., Loge, F.J., Tchobanoglous, G., Darby, J.L., 1996. Ultraviolet disinfection of filtrated activated sludge effluent for reuse applications. Water Environ. Res. 68 (2), 152–161. Bryan, E.H., Dick, R.I., Swinwood, J.F., Chairman, P.K., Carlos, D.A., Gerry, H., Waite, T., 1992. Radiation Energy Treatment of Water, Wastewater and Sludge. American Society of Civil Engineers, New York. Ciebin, B.W., Brodsky, M.H., Eddington, R., Horsnek, G., Choney, A., Palmateer, G., Ley, A., Joshi, R., Shears, G., 1995. Comparative evaluation of modified m-FC and m-TEC media for membrane filter enumeration of E. coli in water. Appl. Environ. Microbiol. 61 (11), 3940–3942. Clesceri, L.S., Greenberg, A.E., Eaton, A.D., (Eds.), 1998. Standard Methods for the Examination of Water and Wastewater, 20th ed., American Public Health Association, Washington, DC. Dietrich, J.P., Basagaoglu, H., Loge, F.J., Ginn, T.R., 2003. Preliminary assessment of transport processes influencing the penetration of chlorine into wastewater particles and the subsequent inactivation of particle-associated organisms. Water Res. 37 (1), 139–149. Emerick, R.W., Loge, F.J., Ginn, T.R., Darby, J.L., 2000. Modeling the inactivation of particle-associated coliform bacteria. Water Environ. Res. 72 (4), 432–438. Farooq, S., Kuruckz, C.N., Waite, D.W., Cooper, W., 1993. Disinfection of waste waters: high energy electron vs gamma irradiation. Water Res. 27 (7), 1177–1184. International Atomic Energy Agency, 1990. Technical Document, Proceedings of the Joint ASCE-IAEA Meeting on Radiation Treatment, Washington, DC, July 1990. International Atomic Energy Agency, 1997. Technical Document, GC(41)/INF/5. Lessel, T., Suess, A., 1984. Ten-year experience in operation of a sludge treatment plant using Gamma irradiation. Radn. Phys. Chem. J. 24, 3–16. Liltved, H., Cripps, S.J., 1999. Removal of particle-associated bacteria by prefiltration and ultraviolet irradiation. Aquacult. Res. 30, 445–450. Loge, F.J., Emerick, R.W., Thompson, D.E., Nelson, D.C., Darby, J.L., 1999. Factors influencing ultraviolet disinfection performance. Part I: light penetration to wastewater particles. Water Environ. Res. 71 (3), 377–381.
ARTICLE IN PRESS 3948
F. Taghipour / Water Research 38 (2004) 3940–3948
Loge, F.J., Emerick, R.W., Ginn, T.R., Darby, J.L., 2001. Association of coliform bacteria with wastewater particles: impact of operational parameters of the activated sludge process. Water Res. 36 (1), 41–48. Oppenheimer, A.J., Jacangelo, J.G., Lane, J.M., Hoagland, J.E., 1997. Testing the Equivalency of Ultraviolet Light and Chlorine for Disinfection of Wastewater to Reclamation Standards. Water Environ. Res. 69, 14–24. Ormeci, B., Linden, K.G., 2002. Comparison of UV and chlorine inactivation of particle and non-particle associated coliform. Wat. Sci. Technol:Wat. Sup. 2 (5-6), 403–410. Qualls, R.G., Flynn, M.P., Johnson, J.D., 1983. The role of suspended particles in ultraviolet disinfection. J. Water Pollut. Control Fed. 55 (10), 1280–1285. Pribil, W., Sommer, R., Appelt, S., Gehringer, P., Eschweiler, H., Cabaj, H.L.A., Haider, T., 2000. Inactivation of Indicator Bacteria and Indicator Viruses by Ionizing Radiation, University of Vienna. Rawat, K.P., Sharma, A., Rao, S.M., 1998. Microbiological and Physical Analysis of Radiation Disinfected Municipal Sewage. Water Res. 32 (3), 737–740. Sakamoto, G., 1997. Clean Water for the 21st Century, Doing More for Less, UV Disinfection For Wastewater Reclamation, Proceedings of the 1997 PNPCA Annual Conference, Seattle, Washington, October, 1997. Scheible, O.K., Casey, M.C., Forndran, A., 1986. Ultraviolet Disinfection of Wastewater from Secondary Effluent and Combined Sewer Overflows. EPA/600/2-86/005, PB86-145182, 1986, US Environmental Protection Agency, Cincinnati, OH. Schmelling, D., 2003. Ultraviolet Disinfection Guidance Manual, EPA 815-D-03-007.
Slifko, T.R., Rose, J.B., Cooper, W.J., Nickelsen, M.J., 1999. Inactivation of Waterborne Microorganisms by a High-energy Electron Beam: a large scale study. International Symposium on Waterborne Pathogens, Proceedings, Milwaukee, WI, USA, August 29–September 1, pp. 112–127. Sommer, R., Pribil, W., Appelt, S., Gehringer, P., Eschweiler, H., Leth, H., Cabaj, A., Haider, T., 2001. Inactivation of bacteriophages in water by means of non-ionizing (UV-253.7 nm) and ionizing (Gamma) radiation: a comparative approach. Water Res. 35 (13), 3109–3116. Task Force on Wastewater Disinfection (Ed.), 1996. Ultraviolet Disinfection, in Wastewater Disinfection. Manual of Practice FD-10, 236-248. Water Environmental Federation, Alexandria, VA. Thompson, J.A., Blatchley, E.R., 2000. Gamma Irradiation for Inactivation of C. parvum, E. coli, and Coliphage MS-2. J. Environ. Eng. 126 (8), 761–768. USEPA, 1992. Ultraviolet Disinfection Technology Assessment. EPA 832-R-92-004, PB92-222868, US Environmental Protection Agency, Washington, DC. Whitby, G.E., Palmateer, G., 1993. The Effect of UV Transmission, Suspended Solids and Photoreactivation on Microorganisms in Wastewater Treated with UV Light. Water Sci. Technol. 27 (3–4), 379–386. Wolfe, R.L., 1990. Ultraviolet Disinfection of Potable Water: Current Technology and Research. Environ. Sci. Technol. 24 (6), 768–773. Zukovs, G., Kollar, J., Monteith, H.D., Ho, K.W.A., Ross, S.A., 1986. Disinfection of Low Quality Wastewaters by Ultraviolet Light Irradiation. J. Water Pollut. Control Fed. 58 (3), 199–206.